A single-celled organism without a nucleus, that is, a prokaryote. At one point a distinction was made between the colorless bacteria and the colored blue-green algae, which were placed among the fungi and algae respectively, but this has been abandoned as artificial. Under the five-kingdom system bacteria are all grouped together under the kingdom Monera. In the three domain system, they are separated out into the Eubacteria including most familiar forms and the Archaea, which contains some extreme forms that are not believed to be closer relatives of the eubacteria than the eukaryotes. Sometimes the word bacterium is restricted to members of the Eubacteria.
Antibiotics kill bacteria or they hamper their growth. Hence bactericidal or bacteriostatic antibiotics.